"quack" in the noun sense
an untrained person who pretends to be a physician and who dispenses medical advice
the harsh sound of a duck
"quack" in the verb sense
utter quacking noises
"The ducks quacked"
act as a medical quack or a charlatan
"quack" in the adjective sense
"a quack doctor"
Source: WordNet® (An amazing lexical database of English)
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